Titanium-containing alloys and associated methods of manufacture

ABSTRACT

Titanium-containing alloys are generally described. The titanium-containing alloys are, according to certain embodiments, nanocrystalline. According to certain embodiments, the titanium-containing alloys have high relative densities. The titanium-containing alloys can be relatively stable, according to certain embodiments. Inventive methods for making titanium-containing alloys are also described herein. The inventive methods for making titanium-containing alloys can involve, according to certain embodiments, sintering nanocrystalline particulates comprising titanium and at least one other metal to form a titanium-containing nanocrystalline alloy.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application No. 62/384,232, filed Sep. 7, 2016, and entitled “Stable Nano-Duplex Titanium-Magnesium Alloys,” which is incorporated herein by reference in its entirety for all purposes.

TECHNICAL FIELD

Titanium-containing alloys and associated methods of manufacture are generally described.

BACKGROUND

Nanocrystalline materials can be susceptible to grain growth. In certain instances, prior sintering techniques for titanium-based alloys have made it difficult to produce nanocrystalline materials, including bulk nanocrystalline materials, that have both small grain sizes and high relative densities. Improved systems and methods, and associated metal alloys, would be desirable.

SUMMARY

Titanium-containing alloys are generally described. The titanium-containing alloys are, according to certain embodiments, nanocrystalline. According to certain embodiments, the titanium-containing alloys have high relative densities. The titanium-containing alloys can be relatively stable, according to certain embodiments. Inventive methods for making titanium-containing alloys are also described herein. The inventive methods for making titanium-containing alloys can involve, according to certain embodiments, sintering nanocrystalline particulates comprising titanium and at least one other metal to form a titanium-containing nanocrystalline alloy. The subject matter of the present invention involves, in some cases, interrelated products, alternative solutions to a particular problem, and/or a plurality of different uses of one or more systems and/or articles.

According to one aspect, inventive metal alloys are provided.

Certain embodiments are related to a sintered nanocrystalline metal alloy, comprising Ti a second metal, wherein Ti is the most abundant metal by atomic percentage in the sintered nanocrystalline metal alloy, and the sintered nanocrystalline metal alloy has a relative density of at least 80%.

According to some embodiments, a sintered nanocrystalline metal alloy comprises Ti and a second metal, wherein the second metal and Ti exhibit a miscibility gap, and the sintered nanocrystalline metal alloy has a relative density of at least 80%.

Some embodiments are related to a bulk nanocrystalline metal alloy comprising Ti and a second metal, wherein Ti is the most abundant metal by atomic percentage in the bulk nanocrystalline metal alloy, and the bulk nanocrystalline metal alloy is substantially stable at a temperature that is greater than or equal to 100° C.

Certain embodiments are related to a bulk nanocrystalline metal alloy comprising Ti and a second metal, wherein Ti is the most abundant metal by atomic percentage in the bulk nanocrystalline metal alloy, and the bulk nanocrystalline metal alloy has an average grain size of less than 300 nm.

According to some embodiments, a metal alloy comprises Ti and Mg, wherein the metal alloy has a relative density of greater than or equal to 80%. In some such embodiments, the metal alloy comprises a nano-duplex structure comprising or consisting of Ti-rich grains and Mg-rich precipitates.

In another aspect, methods of forming metal alloys are provided.

In accordance with some embodiments, a method of forming a nanocrystalline metal alloy comprises sintering a plurality of nanocrystalline particulates to form the nanocrystalline metal alloy, wherein at least some of the nanocrystalline particulates comprise Ti and a second metal, and Ti is the most abundant metal by atomic percentage in at least some of the nanocrystalline particulates.

According to certain embodiments, a method of forming a nanocrystalline metal alloy comprises sintering a plurality of nanocrystalline particulates to form the nanocrystalline metal alloy; wherein at least some of the nanocrystalline particulates comprise Ti and a second metal; and sintering the plurality of nanocrystalline particulates involves heating the nanocrystalline particulates to a first sintering temperature that is greater than or equal to 300° C. and less than or equal to 850° C. for a sintering duration greater than or equal to 10 minutes and less than or equal to 24 hours.

In some embodiments, a method of forming a nanocrystalline metal alloy comprises sintering a plurality of nanocrystalline particulates to form the nanocrystalline metal alloy; wherein at least some of the nanocrystalline particulates comprise Ti and a second metal; and sintering the plurality of nanocrystalline particulates involves heating the nanocrystalline particulates such that the nanocrystalline particulates are not at a temperature of greater than or equal to 1200° C. for more than 24 hours.

In accordance with certain embodiments, a method of forming a nanocrystalline metal alloy comprises sintering a plurality of nanocrystalline particulates to form the nanocrystalline metal alloy; wherein at least some of the nanocrystalline particulates comprise Ti and a second metal; Ti is the most abundant metal by atomic percentage in at least some of the nanocrystalline particulates; and the sintering comprises heating the nanocrystalline particulates to a first sintering temperature lower than a second sintering temperature needed for sintering Ti in the absence of the second metal.

In some embodiments, a method of forming a nanocrystalline metal alloy comprises sintering a plurality of nanocrystalline particulates to form the nanocrystalline metal alloy; wherein at least some of the nanocrystalline particulates comprise Ti and a second metal; and the second metal and Ti exhibit a miscibility gap.

In certain embodiments, a method of forming a nanocrystalline metal alloy comprises sintering a plurality of nanocrystalline particulates to form the nanocrystalline metal alloy, wherein at least some of the nanocrystalline particulates comprise Ti and a second metal; Ti is the most abundant metal by atomic percentage in at least some of the nanocrystalline particulates; and the nanocrystalline metal alloy has a relative density of at least 80%.

In accordance with some embodiments, a method of forming a metal alloy comprises sintering powder comprising Ti and Mg to produce the metal alloy, wherein the metal alloy has a relative density of greater than or equal to 80%. In some such embodiments, the method further comprises milling powders of elemental Ti and Mg. For example, powders of elemental Ti and Mg can be mixed and milled (e.g., to achieve supersaturation and a decrease of the grain size to the nanometer scale). In some such embodiments, the powders can be compressed prior to sintering. According to some such embodiments, a nano-duplex structure consisting of Ti-rich grains and Mg-rich precipitates is developed.

Other advantages and novel features of the present invention will become apparent from the following detailed description of various non-limiting embodiments of the invention when considered in conjunction with the accompanying figures. In cases where the present specification and a document incorporated by reference include conflicting and/or inconsistent disclosure, the present specification shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. In the figures:

FIGS. 1A-1C are schematic diagrams showing a sintering process, according to certain embodiments;

FIG. 2 is a plot of the enthalpy of segregation, ΔH^(seg) (kJ/mol) vs. the enthalpy of mixing, ΔH^(mix) (kJ/mol) of various metals with titanium, according to some embodiments;

FIG. 3 shows a series of x-ray diffraction (XRD) spectra for nanocrystalline powder samples, according to certain embodiments;

FIG. 4 is a plot of grain sizes and lattice parameters of nanocrystalline powders that contained titanium and 10 at. % Mg, 20 at. % Mg, and 30 at. % Mg, according to some embodiments;

FIG. 5 is, in accordance with some embodiments, a series of TEM images and corresponding electron diffraction patterns for nanocrystalline powders that contained titanium and 10 at. % Mg, 20 at. % Mg, and 30 at. % Mg;

FIG. 6 is, according to some embodiments, an electron diffraction pattern from a TEM of Ti-20 at. % Mg;

FIGS. 7A-7B are a set of photographs of samples that were powders containing different atomic percentages of titanium and magnesium that had been cold pressed and covered with a tantalum (Ta) foil (FIG. 7A) or a copper (Cu) tube (FIG. 7B), according to certain embodiments;

FIG. 8 is, according to some embodiments, a plot of relative density as a function of applied load;

FIG. 9 is, in accordance with some embodiments, a plot of the change in relative density as a function of sintering temperature;

FIGS. 10A-10C show scanning transmission electron microscopy-energy dispersive x-ray spectroscopy (STEM-EDS) images for Ti-20 at % Mg after sintering at 500° C. for 8 h, according to certain embodiments;

FIG. 11 is an XRD plot before (dotted) and after (solid lines) sintering, according to certain embodiments;

FIG. 12 is an STEM image of a metal alloy after sintering, according to some embodiments; and

FIG. 13 is an STEM image of a metal alloy after sintering, according to certain embodiments.

DETAILED DESCRIPTION

Nanocrystalline metals have certain advantages over their microcrystalline counterparts due to the large volume fraction of grain boundaries. As one example, nanocrystalline alloys generally have remarkably higher tensile strength. However, nanocrystalline metals have primarily been processed as thin films, as retaining nanoscale grains in processing a bulk material is much more difficult.

This disclosure is generally directed to metal alloys comprising titanium. The metal alloys comprising the titanium are, according to certain embodiments, nanocrystalline metal alloys. Certain of the metal alloys described herein can have high relative densities while maintaining their nanocrystalline character. In addition, according to certain embodiments, the metal alloys can be bulk metal alloys. Certain of the metal alloys described herein are stable against grain growth.

Inventive methods for making titanium-containing alloys are also described herein. For example, certain embodiments are directed to sintering methods in which the sintering is achieved at relatively low temperatures and/or over a relatively short period of time. According to some embodiments, and as described in more detail below, the sintering can be performed such that undesired grain growth is limited or eliminated (e.g., via the selection of materials and/or sintering conditions). Certain embodiments are directed to the recognition that one can sinter titanium-containing materials over relatively short times and/or at relatively low temperatures while maintaining nanocrystallinity.

Certain of the embodiments described herein can provide advantages relative to prior articles, systems, and methods. For example, according to certain (although not necessarily all) embodiments, the titanium-containing metal alloys can have high strength, high hardness, and/or high resistance to grain growth. According to some (although not necessarily all) embodiments, the methods for forming metal alloys described herein can make use of relatively small amounts of energy, for example, due to the relatively short sintering times and/or the relatively low sintering temperatures that are employed.

As noted above, certain embodiments are related to inventive metal alloys. The metal alloys comprise, according to certain embodiments, titanium and at least one other metal.

According to certain embodiments, the metal alloy comprises titanium (Ti). The metal alloy can contain, according some embodiments, a relatively large amount of titanium. For example, in some embodiments, Ti is the most abundant metal by atomic percentage in the metal alloy. (Atomic percentages are abbreviated herein as “at. %” or “at %”.) According to certain embodiments, Ti is present in the metal alloy in an amount of at least 50 at. %, at least 55 at. %, at least 60 at. %, at least 70 at. %, at least 80 at. %, at least 90 at. %, or at least 95 at. %. In some embodiments, Ti is present in the metal alloy in an amount of up to 96 at. %, up to 97 at. %, up to 98 at. %, or more. Combinations of these ranges are also possible. Other values are also possible.

The metal alloys described herein can comprise a second metal. The phrase “second metal” is used herein to describe any metal element that is not Ti. The term “element” is used herein to refer to an element as found on the Periodic Table. “Metal elements” are those found in Groups 1-12 of the Periodic Table except hydrogen (H); Al, Ga, In, Tl, and Nh in Group 13 of the Periodic Table; Sn, Pb, and Fl in Group 14 of the Periodic Table; Bi and Mc in Group 15 of the Periodic Table; Po and Lv in Group 16 of the Periodic Table; the lanthanides; and the actinides. In some embodiments, the second metal is a refractory metal element (e.g., Nb, Ta, Mo, W, and/or Re). In some embodiments, the second metal is a transition metal (i.e., any of those in Groups 3-12 of the Periodic Table). In some embodiments, the second metal is a lanthanide (an element with the atomic number 57-71, inclusive). In some embodiments, the second metal is a rare earth element, e.g. Scandium (Sc), Yttrium (Y), or a lanthanide. In some embodiments, the second metal is an actinide (an element with the atomic number 89-103, inclusive). According to certain embodiments, the second metal is selected from the group consisting of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), scandium (Sc), yttrium (Y), Lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), Actinium (Ac), thorium (Th), protactinium (Pa), uranium (U), neptunium (Np), plutonium (Pu), americium (Am), curium (Cm), berkelium (Bk), californium (CO, einsteinium (Es), fermium (Fm), mendelevium (Md), nobelium (No), and lawrencium (Lr), zirconium (Zr), hafnium (Hf), rutherfordium (Rf), vanadium (V), niobium (Nb), tantalum (Ta), dubnium (Db), chromium (Cr), molybdenum (Mo), tungsten (W), seaborgium (Sg), manganese (Mn), technetium (Tc), rhenium (Re), bohrium (Bh), iron (Fe), ruthenium (Ru), osmium (Os), hassium (Hs), cobalt (Co), rhodium (Rh), iridium (Ir), meitnerium (Mt), nickel (Ni), palladium (Pd), platinum (Pt), darmstadtium (Ds), copper (Cu), silver (Ag), gold (Au), roentgenium (Rg), zinc (Zn), cadmium (Cd), mercury (Hg), copernicium (Cn), aluminum (Al), gallium (Ga), indium (In), thallium (Tl), nihonium (Nh), tin (Sn), lead (Pb), flerovium (Fl), bismuth (Bi), moscovium (Mc), polonium (Po), and livermorium (Lv). The metal alloy can comprise, in some embodiments, combinations of two or more of these.

According to certain embodiments, the second metal comprises an alkaline earth metal. The phrase “alkaline earth metal” is used herein to describe the elements in Group 2 of the Periodic Table (i.e., Be, Mg, Ca, Sr, Ba, and Ra). In certain embodiments, the second metal is selected from the group consisting of Mg, La, Y, Th, Sc, Cr, Ag, Fe, Mn, Cu, and Li. In some embodiments, the second metal is Mg.

According to certain embodiments, the second metal and Ti exhibit a miscibility gap. Two elements are said to exhibit a “miscibility gap” when the phase diagram of those two elements includes a region in which the mixture of the two elements exists as two or more phases. In some embodiments in which the second metal and Ti exhibit a miscibility gap, the second metal and Ti can be present in the metal alloy among at least two phases.

In some embodiments, Ti is at least partially soluble in the second metal. For example, in some embodiments, Ti and the second metal are in a solid solution.

The second metal may be present in the metal alloy in a variety of suitable percentages. According to certain embodiments, the second metal is present in the metal alloy in an amount of less than or equal to 40 at. %, less than or equal to 35 at. %, less than or equal to 32 at. %, less than or equal to 30 at. %, less than or equal to 25 at. %, less than or equal to 22 at. %, less than or equal to 20 at. %, less than or equal to 15 at. %, or less than or equal to 12 at. %. In some embodiments, the second metal is present in the metal alloy in an amount of at least 1 at. %, at least 2 at. %, at least 3 at. %, at least 4 at. %, at least 5 at. %, at least 6 at. %, at least 7 at. %, at least 8 at. %, at least 9 at. %, at least 10 at. %, or more. Combinations of these ranges are also possible. For example, in some embodiments, the second metal is present in the metal alloy in an amount of from 1 at. % to 40 at. % of the metal alloy. In some embodiments, the second metal is present in the metal alloy in an amount of from 8 at. % to 32 at. % of the metal alloy. Other values are also possible.

In some embodiments, the second metal may be an activator element, relative to Ti. Activator elements are those elements that increase the rate of sintering of a material, relative to sintering rates that are observed in the absence of the activator element but under otherwise identical conditions. Activator elements are described in more detail below.

In certain embodiments, the second metal may be a stabilizer element, relative to Ti. Stabilizer elements are those elements that reduce the rate of grain growth of a material, relative to grain growth rates that are observed in the absence of the stabilizer element but under otherwise identical conditions. Stabilizer elements are described in more detail below. In some embodiments, the second metal may be both a stabilizer element and an activator element.

According to certain embodiments, the second metal (e.g., for forming an alloy with Ti) can be selected based on one or more of the following conditions:

1. thermodynamic stabilization of the nanocrystalline grain size;

2. phase separation region, which is extended above the sintering temperature;

3. second (e.g., solute) element with lower melting temperature; and/or

4. solubility of the Ti in the precipitated second phase.

According to some embodiments, the second metal (e.g., Mg) forms a nano-duplex structure with the Ti. For example, in some embodiments, the metal alloy comprises a nano-duplex structure consisting of Ti-rich grains and Mg-rich precipitates. In some embodiments, nanocrystalline structure with grain sizes of around 110 nm can be maintained even after 8 hours at 500° C. (which is 84% of the melting temperature for Mg and 30% for Ti). According to some embodiments, high relative densities can be achieved for Ti-20 at. % Mg and Ti-30 at. % Mg.

In some embodiments, the metal alloy comprises only Ti and the second metal (i.e., Ti and the second metal without additional metals or other elements). In other embodiments, the metal alloy comprises Ti, the second metal, and a third element. For example, in some embodiments, the metal alloy comprises a third metal (in addition to Ti and the second metal). The phrase “third metal” is used herein to describe a metal that is not Ti and that is not the second metal.

The third metal may be present in the metal alloy in a variety of suitable percentages. According to certain embodiments, the third metal is present in the metal alloy in an amount of less than or equal to 40 at. %, less than or equal to 35 at. %, less than or equal to 32 at. %, less than or equal to 30 at. %, less than or equal to 25 at. %, less than or equal to 22 at. %, less than or equal to 20 at. %, less than or equal to 15 at. %, or less than or equal to 12 at. %. In some embodiments, the third metal is present in the metal alloy in an amount of at least 1 at. %, at least 2 at. %, at least 3 at. %, at least 4 at. %, at least 5 at. %, at least 6 at. %, at least 7 at. %, at least 8 at. %, at least 9 at. %, at least 10 at. %, or more. Combinations of these ranges are also possible. Other values are also possible.

According to certain embodiments, the third metal may be a stabilizer element, an activator element, or both a stabilizer element and an activator element.

In some embodiments, the metal alloy comprises Ti and at least one of Mg, La, Y, Th, Sc, Cr, Ag, Fe, Mn, Cu, and Li. In some embodiments, the metal alloy comprises Ti, Mg, and at least one of La, Y, Th, Sc, Cr, Ag, Fe, Mn, Cu, and Li.

According to certain embodiments, the total amount of all metal elements in the metal alloy that are not Ti (e.g., the second metal, the optional third metal, and any additional optional metals) makes up less than 50 at. %, less than or equal to 40 at. %, less than or equal to 35 at. %, less than or equal to 32 at. %, less than or equal to 30 at. %, less than or equal to 25 at. %, less than or equal to 22 at. %, less than or equal to 20 at. %, less than or equal to 15 at. %, or less than or equal to 12 at. % of the metal alloy. In some embodiments, the total amount of all metal elements in the metal alloy that are not Ti (e.g., the second metal, the optional third metal, and any additional optional metals) makes up at least 1 at. %, at least 2 at. %, at least 3 at. %, at least 4 at. %, at least 5 at. %, at least 6 at. %, at least 7 at. %, at least 8 at. %, at least 9 at. %, at least 10 at. %, or more. Combinations of these ranges are also possible. Other values are also possible.

According to certain embodiments, the metal alloys are nanocrystalline metal alloys. Nanocrystalline materials generally refer to materials that comprise at least some grains with a grain size smaller than or equal to 1000 nm. In some embodiments, the nanocrystalline material comprises grains with a grain size smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, or smaller than or equal to 20 nm. Accordingly, in the case of metal alloys, nanocrystalline metal alloys are metal alloys that comprise grains with a grain size smaller than or equal to 1000 nm. In some embodiments, the nanocrystalline metal alloy comprises grains with a grain size smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, or smaller than or equal to 20 nm. Other values are also possible.

The “grain size” of a grain generally refers to the largest dimension of the grain. The largest dimension may be a diameter, a length, a width, or a height of a grain, depending on the geometry thereof. According to certain embodiments, the grains may be spherical, cubic, conical, cylindrical, needle-like, or any other suitable geometry.

According to certain embodiments, a relatively large percentage of the volume of the metal alloy is made up of small grains. For example, in some embodiments, at least 50%, at least 75%, at least 90%, at least 95%, at least 99%, or substantially all of the volume of the metal alloy is made up of grains having grain sizes of smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, or smaller than or equal to 20 nm. Other values are also possible.

According to certain embodiments, the metal alloy may have a relatively small average grain size. The “average grain size” of a material (e.g., a metal alloy) refers to the number average of the grain sizes of the grains in the material. According to certain embodiments, the metal alloy (e.g., a bulk and/or nanocrystalline metal alloy) has an average grain size of smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, or smaller than or equal to 20 nm. In certain embodiments, the metal alloy has an average grain size of as little as 25 nm, as little as 10 nm, as little as 1 nm, or smaller. Combinations of these ranges are also possible. Other values are also possible.

According to certain embodiments, at least one cross-section of the metal alloy that intersects the geometric center of the metal alloy has a small volume-average cross-sectional grain size. The “volume-average cross-sectional grain size” of a given cross-section of a metal alloy is determined by obtaining the cross-section of the object, tracing the perimeter of each grain in an image of the cross-section of the object (which may be a magnified image, such as an image obtained from a transmission electron microscope), and calculating the circular-equivalent diameter, D_(i), of each traced grain cross-section. The “circular-equivalent diameter” of a grain cross-section corresponds to the diameter of a circle having an area (A, as determined by A=πr²) equal to the cross-sectional area of the grain in the cross-section of the object. The volume-average cross-sectional grain size (G_(CS,avg)) is calculated as:

$G_{{CS},{avg}} = \left( \frac{\sum\limits_{i = 1}^{i = n}D_{i}^{3}}{n} \right)^{1/3}$

where n is the number of grains in the cross-section and D_(i) is the circular-equivalent diameter of grain i.

According to certain embodiments, at least one cross-section of the metal alloy that intersects the geometric center of the metal alloy has a volume-average cross-sectional grain size of smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, or smaller than or equal to 20 nm. In certain embodiments, at least one cross-section of the metal alloy that intersects the geometric center of the metal alloy has a volume-average cross-sectional grain size of as small as 25 nm, as small as 10 nm, as small as 1 nm, or smaller. Combinations of these ranges are also possible. Other values are also possible.

According to certain embodiments, at least one cross-section of the metal alloy (that, optionally, intersects the geometric center of the metal alloy) has a volume-average cross-sectional grain size of smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, or smaller than or equal to 20 nm (and/or as small as 25 nm, as small as 10 nm, as small as 1 nm, or smaller); and at least a second cross-section of the metal alloy that is orthogonal to the first cross section (that, optionally, intersects the geometric center of the metal alloy) has a volume-average cross-sectional grain size of smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, or smaller than or equal to 20 nm (and/or as small as 25 nm, as small as 10 nm, as small as 1 nm, or smaller). Other values are also possible.

According to certain embodiments, at least one cross-section of the metal alloy (that, optionally, intersects the geometric center of the metal alloy) has a volume-average cross-sectional grain size of smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, or smaller than or equal to 20 nm (and/or as small as 25 nm, as small as 10 nm, as small as 1 nm, or smaller); at least a second cross-section of the metal alloy that is orthogonal to the first cross section (that, optionally, also intersects the geometric center of the metal alloy, or otherwise) has a volume-average cross-sectional grain size of smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, or smaller than or equal to 20 nm (and/or as small as 25 nm, as small as 10 nm, as small as 1 nm, or smaller); and at least a third cross-section of the metal alloy that is orthogonal to the first cross-section and that is orthogonal to the second cross-section (that, optionally, also intersects the geometric center of the metal alloy) has a volume-average cross-sectional grain size of smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, or smaller than or equal to 20 nm (and/or as small as 25 nm, as small as 10 nm, as small as 1 nm, or smaller).

In some embodiments, the metal alloy comprises grains that are relatively equiaxed. In certain embodiments, at least a portion of the grains within the metal alloy have aspect ratios of less than or equal to 2, less than or equal to 1.8, less than or equal to 1.6, less than or equal to 1.4, less than or equal to 1.3, less than or equal to 1.2, or less than or equal to 1.1 (and, in some embodiments, down to 1). The aspect ratio of a grain is calculated as the maximum cross-sectional dimension of the grain which intersects the geometric center of the grain, divided by the largest dimension of the grain that is orthogonal to the maximum cross-sectional dimension of the grain. The aspect ratio of a grain is expressed as a single number, with 1 corresponding to an equiaxed grain. In some embodiments, the number average of the aspect ratios of the grains in the metal alloy is less than or equal to 2, less than or equal to 1.8, less than or equal to 1.6, less than or equal to 1.4, less than or equal to 1.3, less than or equal to 1.2, or less than or equal to 1.1 (and, in some embodiments, down to 1).

Without wishing to be bound by any particular theory, it is believed that relatively equiaxed grains may be present when the metal alloy is produced in the absence (or substantial absence) of applied pressure (e.g., via a pressureless or substantially pressureless sintering process).

In certain embodiments, the metal alloy comprises a relatively low cross-sectional average grain aspect ratio. In some embodiments, the cross-sectional average grain aspect ratio in the metal alloy is less than or equal to 2, less than or equal to 1.8, less than or equal to 1.6, less than or equal to 1.4, less than or equal to 1.3, less than or equal to 1.2, or less than or equal to 1.1 (and, in some embodiments, down to 1). The “cross-sectional average grain aspect ratio” of a metal alloy is said to fall within a particular range if at least one cross-section of the metal alloy that intersects the geometric center of the metal alloy is made up of grain cross-sections with an average aspect ratio falling within that range. For example, the cross-sectional average grain aspect ratio of a metal alloy would be less than 2 if the metal alloy includes at least one cross-section that intersects the geometric center of the metal alloy and in which the cross-section is made up of grain cross-sections with an average aspect ratio of less than 2. To determine the average aspect ratio of the grain cross-sections from which the cross-section of the metal alloy is made up (also referred to herein as the “average aspect ratio of grain cross-sections”), one obtains the cross-section of the metal alloy, traces the perimeter of each grain in an image of the cross-section of the metal alloy (which may be a magnified image, such as an image obtained from a transmission electron microscope), and calculates the aspect ratio of each traced grain cross-section. The aspect ratio of a grain cross-section is calculated as the maximum cross-sectional dimension of the grain cross-section (which intersects the geometric center of the grain cross-section), divided by the largest dimension of the grain cross-section that is orthogonal to the maximum cross-sectional dimension of the grain cross-section. The aspect ratio of a grain cross-section is expressed as a single number, with 1 corresponding to an equiaxed grain cross-section. The average aspect ratio of the grain cross-sections from which the cross-section of the metal alloy is made up (AR_(avg)) is calculated as a number average:

${AR}_{avg} = \frac{\sum\limits_{i = 1}^{i = n}{AR}_{i}}{n}$

where n is the number of grains in the cross-section and AR_(i) is the aspect ratio of the cross-section of grain i.

According to certain embodiments, a metal alloy having a cross-sectional average grain aspect ratio falling within a particular range (e.g., any of the ranges described elsewhere herein) has a first cross-section intersecting the geometric center of the metal alloy and having an average aspect ratio of grain cross-sections falling within that range, and at least a second cross-section-orthogonal to the first cross-section-intersecting the geometric center of the metal alloy and having an average aspect ratio of grain cross-sections falling within that range. For example, according to certain embodiments, a metal alloy having a cross-sectional average grain aspect ratio of less than 2 includes a cross-section that intersects the geometric center of the metal alloy having an average aspect ratio of grain cross-sections of less than 2 and at least a second cross-section-orthogonal to the first cross-section-intersecting the geometric center of the metal alloy and having an average aspect ratio of grain cross-sections of less than 2.

According to certain embodiments, a metal alloy having a cross-sectional average grain aspect ratio falling within a particular range (e.g., any of the ranges described elsewhere herein) has a first cross-section intersecting the geometric center of the metal alloy and having an average aspect ratio of grain cross-sections falling within that range; a second cross-section-orthogonal to the first cross-section-intersecting the geometric center of the metal alloy and having an average aspect ratio of grain cross-sections falling within that range; and at least a third cross-section-orthogonal to the first cross-section and the second cross-section-intersecting the geometric center of the metal alloy and having an average aspect ratio of grain cross-sections falling within that range. For example, according to certain embodiments, a metal alloy having a cross-sectional average grain aspect ratio of less than 2 includes a first cross-section that intersects the geometric center of the metal alloy having an average aspect ratio of grain cross-sections of less than 2, a second cross-section-orthogonal to the first cross-section-intersecting the geometric center of the metal alloy and having an average aspect ratio of grain cross-sections of less than 2, and at least a third cross-section-orthogonal to the first cross-section and the second cross-section-intersecting the geometric center of the metal alloy and having an average aspect ratio of grain cross-sections of less than 2.

According to certain embodiments, the grains within the metal alloy can be both relatively small and relatively equiaxed. For example, according to certain embodiments, at least one cross-section (and, in some embodiments, at least a second cross-section that is orthogonal to the first cross-section and/or at least a third cross-section that is orthogonal to the first and second cross-sections) can have a volume-average cross-sectional grain size and an average aspect ratio of grain cross-sections falling within any of the ranges outlined above or elsewhere herein.

The metal alloy can, according to certain embodiments, be a bulk metal alloy (e.g., a bulk nanocrystalline metal alloy). A “bulk metal alloy” is a metal alloy that is not in the form of a thin film. In certain embodiments, the bulk metal alloy has a smallest dimension of at least 1 micron. In some embodiments, the bulk metal alloy has a smallest dimension of at least 10 microns, at least 25 microns, at least 50 microns, at least 100 microns, at least 500 microns, at least 1 millimeter, at least 1 centimeter, at least 10 centimeters, at least 100 centimeters, or at least 1 meter. Other values are also possible. According to certain embodiments, the metal alloy is not in the form of a coating.

In certain embodiments, the metal alloy occupies a volume of at least 1 mm³, at least 5 mm³, at least 10 mm³, at least 0.1 cm³, at least 0.5 cm³, at least 1 cm³, at least 10 cm³, at least 100 cm³, or at least 1 m³. Other values are also possible.

According to certain embodiments, the metal alloy comprises multiple phases. For example, in some embodiments, the metal alloy is a dual-phase metal alloy.

In some embodiments, the metal alloy has a high relative density. The term “relative density” refers to the ratio of the experimentally measured density of the metal alloy and the maximum theoretical density of the metal alloy. The “relative density” (ρ_(rel)) is expressed as a percentage, and is calculated as:

$\rho_{rel} = {\frac{\rho_{measured}}{\rho_{maximum}} \times 100\%}$

wherein ρ_(measured) is the experimentally measured density of the metal alloy and ρ_(maximum) is the maximum theoretical density of an alloy having the same composition as the metal alloy.

In some embodiments, the metal alloy (e.g., a sintered metal alloy, a nanocrystalline metal alloy, and/or a bulk metal alloy) has a relative density of at least 80%, at least 85%, at least 90%, at least 92%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% (and/or, in certain embodiments, up to 99.8%, up to 99.9%, or more). In some embodiments, the nanocrystalline alloy has a relative density of 100%. Other values are also possible.

According to certain embodiments, the metal alloy is fully dense. As utilized herein, the term “fully dense” (or “full density”) refers to a material with a relative density of at least 98%. According to certain embodiments, the relative density of the metal alloy may impact other material properties of the metal alloy. Thus, by controlling the relative density of the metal alloy, other material properties of the metal alloy may be controlled.

According to certain embodiments, metal alloys described herein can be stable at relatively high temperatures. A metal alloy is said to be “substantially stable” at a particular temperature when the metal alloy includes at least one cross-section intersecting the geometric center of the alloy in which the volume-average cross-sectional grain size (described above) of the cross-section does not increase by more than 20% (relative to the original volume-average cross-sectional grain size) when the metal alloy is heated to that temperature for 24 hours in an argon atmosphere. One of ordinary skill in the art would be capable of determining whether a metal alloy is stable at a particular temperature by taking a cross-section of the article, determining the volume-average cross-sectional grain size of the cross-section at 25° C., heating the cross-section to the particular temperature for 24 hours in an argon atmosphere, allowing the cross-section to cool back to 25° C., and determining—post-heating—the volume-average cross-sectional grain size of the cross-section. The metal alloy would be said to be stable if the volume-average cross-sectional grain size of the cross-section after the heating step is less than 120% of the volume-average cross-sectional grain size of the cross-section prior to the heating step. According to certain embodiments, a metal alloy that is stable at a particular temperature includes at least one cross-section intersecting the geometric center of the metal alloy in which the volume-average cross-sectional grain size of the cross-section does not increase by more than 15%, more than 10%, more than 5%, or more than 2% (relative to the original volume-average grain size) when the object is heated to that temperature for 24 hours in an argon atmosphere.

In some embodiments, the metal alloy is substantially stable at at least one temperature that is greater than or equal to 100 degrees Celsius (° C.). In certain embodiments, the metal alloy is substantially stable at at least one temperature that is greater than or equal to 200° C., greater than or equal to 300° C., greater than or equal to 400° C., greater than or equal to 500° C., greater than or equal to 600° C., greater than or equal to 700° C., greater than or equal to 800° C., greater than or equal to 900° C., greater than or equal to 1000° C., greater than or equal to 1100° C., greater than or equal to 1200° C., greater than or equal to 1300° C., or greater than or equal to 1400° C. Other ranges are also possible.

Certain of the metal alloys described herein are sintered metal alloys. Exemplary sintering methods that may be used to produce metal alloys according to the present disclosure are described in more detail below.

Also described herein are inventive methods of forming metal alloys (e.g., sintered metal alloys, bulk metal alloys, and/or nanocrystalline metal alloys). Certain of the inventive methods described herein can be used to form the inventive metal alloys described above and elsewhere herein. For example, certain of the methods described herein can be used to form nanocrystalline metal alloys, for example, including any of the grain sizes and/or grain size distributions described above or elsewhere herein. Certain of the methods described herein can be used to form metal alloys having high relative densities, including any of the relative densities described above or elsewhere herein. Certain of the methods described herein can be used to form bulk nanocrystalline metal alloys, for example, having any of the sizes described above or elsewhere herein. Certain of the methods described herein can be used to form metal alloys that are stable, for example, having any of the stabilities (e.g., against grain growth) described above or elsewhere herein.

In some embodiments, a metal alloy is formed by sintering a plurality of particulates. The shape of the particulates may be, for example, spherical, cubical, conical, cylindrical, needle-like, irregular, or any other suitable geometry. In some embodiments, at least some (e.g., at least 50%, at least 75%, at least 90%, or at least 95%) of the particulates are single crystals. In certain embodiments, at least some (e.g., at least 50%, at least 75%, at least 90%, or at least 95%) of the particulates are polycrystalline.

The particulates that are sintered can be, according to certain embodiments, nanocrystalline particulates. The nanocrystalline particulates can comprise, according to certain embodiments, grains with a grain size smaller than or equal to 1000 nm, smaller than or equal to 900 nm, smaller than or equal to 800 nm, smaller than or equal to 700 nm, smaller than or equal to 600 nm, smaller than or equal to 500 nm, smaller than or equal to 400 nm, smaller than or equal to 300 nm, smaller than or equal to 200 nm, smaller than or equal to 150 nm, smaller than or equal to 125 nm, smaller than or equal to 100 nm, smaller than or equal to 50 nm, smaller than or equal to 40 nm, smaller than or equal to 30 nm, or smaller than or equal to 20 nm. According to certain embodiments, at least some of the nanocrystalline particulates have a grain size of smaller than or equal to 50 nm. In some embodiments, at least some of the nanocrystalline particulates have a grain size of greater than or equal to 5 nm and smaller than or equal to 25 nm. In some embodiments, at least some of the nanocrystalline particulates have a grain size of greater than or equal to 10 nm and smaller than or equal to 20 nm.

According to certain embodiments, at least some of the nanocrystalline particulates comprise Ti and/or a second metal. In some embodiments, one portion of the nanocrystalline particulates is made up of Ti while another portion of the nanocrystalline particulates are made up of the second metal. In certain embodiments, at least some of the nanocrystalline particulates comprise both Ti and the second metal.

In some embodiments, Ti is the most abundant metal by atomic percentage in at least some of the nanocrystalline particulates. In some embodiments, at least some of the particulates contain Ti in an amount of at least 50 at. %, at least 55 at. %, at least 60 at. %, at least 70 at. %, at least 80 at. %, at least 90 at. %, or at least 95 at. %. In some embodiments, at least some of the particulates contain Ti in an amount of up to 96 at. %, up to 97 at. %, up to 98 at. %, or more. Combinations of these ranges are also possible. Other values are also possible.

In some embodiments, Ti is the most abundant metal by atomic percentage in the particulate material. According to certain embodiments, the total amount of Ti present in the particulate material is at least 50 at. %, at least 55 at. %, at least 60 at. %, at least 70 at. %, at least 80 at. %, at least 90 at. %, or at least 95 at. % of the particulate material. In some embodiments, the total amount of Ti present in the particulate material is up to 96 at. %, up to 97 at. %, up to 98 at. %, or more of the particulate material. Combinations of these ranges are also possible. Other values are also possible.

The second metal can be, for example, any of the second metals described above.

In some embodiments, at least a portion of the particulates include the second metal in an amount of less than or equal to 40 at. %, less than or equal to 35 at. %, less than or equal to 32 at. %, less than or equal to 30 at. %, less than or equal to 25 at. %, less than or equal to 22 at. %, less than or equal to 20 at. %, less than or equal to 15 at. %, or less than or equal to 12 at. %. In some embodiments, at least a portion of the particulates include the second metal in an amount of at least 1 at. %, at least 2 at. %, at least 3 at. %, at least 4 at. %, at least 5 at. %, at least 6 at. %, at least 7 at. %, at least 8 at. %, at least 9 at. %, at least 10 at. %, or more. Combinations of these ranges are also possible. For example, in some embodiments, at least a portion of the particulates include the second metal in an amount of from 1 at. % to 40 at. % of the particulate material. In some embodiments, at least a portion of the particulates include the second metal in an amount of from 8 at. % to 32 at. % of the particulate material. Other values are also possible.

In some embodiments, the total amount of the second metal in the particulate material is less than or equal to 40 at. %, less than or equal to 35 at. %, less than or equal to 32 at. %, less than or equal to 30 at. %, less than or equal to 25 at. %, less than or equal to 22 at. %, less than or equal to 20 at. %, less than or equal to 15 at. %, or less than or equal to 12 at. % of the particulate material. In some embodiments, the total amount of the second metal in the particulate material is at least 1 at. %, at least 2 at. %, at least 3 at. %, at least 4 at. %, at least 5 at. %, at least 6 at. %, at least 7 at. %, at least 8 at. %, at least 9 at. %, at least 10 at. %, or more of the particulate material. Combinations of these ranges are also possible. For example, in some embodiments, the total amount of the second metal present in the particulate material is from 1 at. % to 40 at. % of the particulate material. In some embodiments, the total amount of the second metal present in the particulate material is from 8 at. % to 32 at. % of the particulate material. Other values are also possible.

According to certain embodiments, at least some of the nanocrystalline particulates are formed by mechanically working a powder comprising the Ti and the second metal. For example, certain embodiments comprise making nanocrystalline particulates, at least in part, by mechanically working a powder including a plurality of Ti particulates and a plurality of second metal particulates. Certain embodiments comprise making nanocrystalline particulates, at least in part, by mechanically working particulates that include both Ti and the second metal.

In embodiments that make use of mechanical working, any appropriate method of mechanical working may be employed to mechanically work a powder and form nanocrystalline particulates. According to certain embodiments, at least some of the nanocrystalline particulates are formed by ball milling a powder comprising the Ti and the second metal. The ball milling process may be, for example, a high energy ball milling process. In a non-limiting exemplary ball milling process, a tungsten carbide or steel milling vial may be employed, with a ball-to-powder ratio of 2:1 to 5:1, and a stearic acid process control agent content of 0.01 wt % to 3 wt %. In some embodiments, the mechanical working may be carried out in the presence of a stearic acid process control agent content of 1 wt %, 2 wt %, or 3 wt %. According to certain other embodiments, the mechanical working is carried out in the absence of a process control agent. Other types of mechanical working may also be employed, including but not limited to, shaker milling and planetary milling. In some embodiments, the mechanical working (e.g., via ball milling or another process) may be performed under conditions sufficient to produce a nanocrystalline particulate comprising a supersaturated phase. Supersaturated phases are described in more detail below.

In certain embodiments, the mechanical working (e.g., ball milling) may be conducted for a time of greater than or equal to 2 hours (e.g., greater than or equal to 4 hours, greater than or equal to 6 hours, greater than or equal to 8 hours, greater than or equal to 10 hours, greater than or equal to 12 hours, greater than or equal to 15 hours, greater than or equal to 20 hours, greater than or equal to 25 hours, greater than or equal to 30 hours, or greater than or equal to 35 hours). In some embodiments, the mechanical working (e.g., ball milling) may be conducted for a time of 1 hour to 35 hours (e.g., 2 hours to 30 hours, 4 hours to 25 hours, 6 hours to 20 hours, 8 hours to 15 hours, or 10 hours to 12 hours). In some cases, if the mechanical working time is too long, the Ti and/or the second metal may be contaminated by the material used to perform the mechanical working (e.g., milling vial material). The amount of the second metal that is dissolved in the Ti may, in some cases, increase with increasing mechanical working (e.g., milling) time. In some embodiments, after the mechanical working step (e.g., ball milling step), a phase rich in the second metal material may be present.

According to certain embodiments, the Ti and the second metal are present in the particulates in a non-equilibrium phase. The particulates may, according to certain embodiments, include a non-equilibrium phase in which the second metal is dissolved in the Ti. In some embodiments, the non-equilibrium phase comprises a solid solution. According to some embodiments, the non-equilibrium phase may be a supersaturated phase comprising the second metal dissolved in the Ti. A “supersaturated phase,” as used herein, refers to a phase in which a material is dissolved in another material in an amount that exceeds the solubility limit. The supersaturated phase can include, in some embodiments, an activator element and/or a stabilizer element forcibly dissolved in the Ti in an amount that exceeds the amount of the activator element and/or the stabilizer element that could be otherwise dissolved in an equilibrium phase of the Ti. For example, in one set of embodiments, the supersaturated phase is a phase that includes an activator element forcibly dissolved in Ti in an amount that exceeds the amount of activator element that could be otherwise dissolved in an equilibrium Ti phase.

In some embodiments, the supersaturated phase may be the only phase present after the mechanical working (e.g., ball milling) process. In certain embodiments, a second phase rich in the second metal may be present after the mechanical working (e.g., ball milling) process. For example, in some cases, a second phase rich in the activator element may be present after mechanical working (e.g., ball milling).

According to certain embodiments, the non-equilibrium phase may undergo decomposition during the sintering of the nanocrystalline particulates (which sintering is described in more detail below). The sintering of the nanocrystalline particulates may cause the formation of a phase rich in the second metal at at least one of the surface and grain boundaries of the nanocrystalline particulates. In some such embodiments, the Ti is soluble in the phase rich in the second metal. The formation of the phase rich in the second metal may be the result of the decomposition of the non-equilibrium phase during the sintering. The phase rich in the second metal may, according to certain embodiments, act as a fast diffusion path for the Ti, enhancing the sintering kinetics and accelerating the rate of sintering of the nanocrystalline particulates. According to some embodiments, the decomposition of the non-equilibrium phase during the sintering of the nanocrystalline particulates accelerates the rate of sintering of the nanocrystalline particulates.

Certain, although not necessarily all, embodiments comprise cold pressing the plurality of nanocrystalline particulates during at least one portion of time prior to the sintering. It has been found that, according to certain embodiments, metal alloys comprising Ti and a second metal (e.g., Ti and Mg) can be compressed such that high relative densities are achieved without the need for simultaneous heating. In some embodiments, the cold pressing comprises compressing of the plurality of nanocrystalline particulates at a force greater than or equal to 300 MPa, greater than or equal to 400 MPa, greater than or equal to 500 MPa, greater than or equal to 750 MPa, greater than or equal to 1000 MPa, greater than or equal to 1500 MPa, greater than or equal to 2000 MPa, or higher. In some embodiments, the cold compression comprises compressing the plurality of nanocrystalline particulates at a force of up to 2500 MPa, or greater. Combinations of these ranges are also possible (e.g., greater than or equal to 300 MPa and less than or equal to 2500 MPa). Other ranges are also possible.

According to certain embodiments, the cold compression is performed at a relatively low temperature. For example, in some embodiments, the cold compression is performed while the particulates are at a temperature of less than or equal to 150° C., less than or equal to 100° C., less than or equal to 75° C., less than or equal to 50° C., less than or equal to 40° C., less than or equal to 35° C., less than or equal to 30° C., less than or equal to 25° C., or less than or equal to 20° C. In some embodiments, the cold compression is performed at a temperature of the surrounding, ambient environment.

As noted above, certain embodiments comprise sintering a plurality of nanocrystalline particulates to form the nanocrystalline metal alloy. Those of ordinary skill in the art are familiar with the process of sintering, which involves applying heat to the material (e.g., particulates) that is to be sintered such that the material becomes a single solid mass.

FIGS. 1A-1C are exemplary schematic diagrams showing a sintering process, according to certain embodiments. In FIG. 1A, a plurality of particulates 100 are shown in the form of spheres (although, as mentioned elsewhere, other shapes could be used). As shown in FIG. 1B, particulates 100 can be arranged such that they contact each other. As shown in FIG. 1C, as the particulates are heated, they agglomerate to form a single solid material 110. During the sintering process, according to certain embodiments, interstices 105 between particulates 100 (shown in FIG. 1B) can be greatly reduced or eliminated, such that a solid having a high relative density is formed (shown in FIG. 1C).

According to certain embodiments, the sintering can be performed when the metal particulates are at a relatively low temperature and/or for a relatively short period of time, while maintaining the ability to form metal alloys having high relative densities, small grain sizes, and/or equiaxed grains.

According to certain embodiments, sintering the plurality of nanocrystalline particulates involves heating the nanocrystalline particulates to a sintering temperature of less than or equal to 1200° C., less than or equal to 1100° C., less than or equal to 1000° C., less than or equal to 900° C., less than or equal to 850° C., less than or equal to 800° C., less than or equal to 750° C., less than or equal to 700° C., less than or equal to 650° C., less than or equal to 600° C., less than or equal to 550° C., less than or equal to 500° C., less than or equal to 450° C., less than or equal to 400° C., or less than or equal to 400° C. According to certain embodiments, sintering the plurality of nanocrystalline particulates involves heating the nanocrystalline particulates to a sintering temperature of greater than or equal to 300° C., greater than or equal to 350° C., greater than or equal to 400° C., greater than or equal to 500° C., greater than or equal to 600° C., greater than or equal to 700° C., or greater than or equal to 900° C. Combinations of these ranges are also possible. For example, in some embodiments, sintering the plurality of nanocrystalline particulates involves heating the nanocrystalline particulates to a sintering temperature that is greater than or equal to 300° C. and less than or equal to 850° C. In some embodiments, sintering the plurality of nanocrystalline particulates involves heating the nanocrystalline particulates to a sintering temperature that is greater than or equal to 300° C. and less than or equal to 450° C. In some embodiments, the temperature of the sintered material is within these ranges for at least 10%, at least 25%, at least 50%, at least 75%, at least 90%, or at least 99% of the sintering time.

According to certain embodiments, sintering the plurality of nanocrystalline particulates involves maintaining the nanocrystalline particulates within the range of sintering temperatures for less than 72 hours, less than 48 hours, less than or equal to 24 hours, less than or equal to 12 hours, less than or equal to 6 hours, less than or equal to 4 hours, less than or equal to 3 hours, less than or equal to 2 hours, or less than or equal to 1 hour (and/or, in some embodiments, for at least 10 minutes, at least 20 minutes, at least 30 minutes, or at least 50 minutes). Combinations of these ranges are also possible. For example, in some embodiments, sintering the plurality of nanocrystalline particulates involves heating the nanocrystalline particulates to a first sintering temperature that is greater than or equal to 300° C. and less than or equal to 850° C. for a sintering duration greater than or equal to 10 minutes and less than or equal to 24 hours. In some embodiments, the sintering comprises heating the nanocrystalline particulates to a temperature greater than or equal to 300° C. and less than or equal to 850° C. for a duration greater than or equal to 20 minutes and less than or equal to 3 hours. In some embodiments, the sintering comprises heating the nanocrystalline particulates to a temperature greater than or equal to 300° C. and less than or equal to 450° C. for a duration greater than or equal to 50 minutes and less than or equal to 2 hours. In certain embodiments, the sintering comprises heating the nanocrystalline particulates to a temperature greater than or equal to 300° C. and less than or equal to 850° C. for a duration greater than or equal to 10 minutes and less than or equal to 2 hours.

According to certain embodiments, during the sintering step, the nanocrystalline particulates are at highly elevated temperatures for only a short period of time (or not at all). In some embodiments, the sintering is performed such that the nanocrystalline particulates are not at a temperature of greater than or equal to 1200° C. (or greater than or equal to 1100° C., greater than or equal to 1000° C., greater than or equal to 900° C., greater than or equal to 800° C., greater than or equal to 700° C., greater than or equal to 600° C., greater than or equal to 500° C., greater than or equal to 400° C., or greater than or equal to 300° C.) for more than 24 hours, more than 12 hours, more than 6 hours, more than 2 hours, more than 1 hour, more than 30 minutes, more than 10 minutes, more than 1 minute, more than 10 seconds, or less. In some embodiments, the sintering is performed such that the nanocrystalline particulates do not exceed a temperature of 1200° C. (or, do not exceed a temperature of 1100° C., do not exceed a temperature of 1000° C., do not exceed a temperature of 900° C., do not exceed a temperature of 800° C., do not exceed a temperature of 700° C., do not exceed a temperature of 600° C., or do not exceed a temperature of 500° C.).

According to certain embodiments, sintering comprises heating the nanocrystalline particulates to a first sintering temperature that is lower than a second sintering temperature needed for sintering Ti in the absence of the second metal. To determine whether such conditions were met, one of ordinary skill in the art would compare the temperature necessary to achieve sintering in the sample containing the Ti and the second metal to the temperature necessary to achieve sintering in a sample containing the Ti without the second metal, but otherwise identical to the sample containing the Ti and the second metal. In some embodiments, the first sintering temperature can be at least 25° C., at least 50° C., at least 100° C., or at least 200° C. lower than the second sintering temperature.

According to certain embodiments, a non-equilibrium phase present in the nanocrystalline particulates (e.g., any of the non-equilibrium phases described above or elsewhere herein) undergoes decomposition during the sintering. In some such embodiments, the decomposition of the non-equilibrium phase accelerates a rate of sintering of the nanocrystalline particulates.

In some embodiments, the sintering further comprises forming a second phase at at least one of a surface and a grain boundary of the nanocrystalline particulates during the sintering. In some such embodiments, Ti is insoluble in the second phase. In some such embodiments, the second phase is rich in the second metal. The term “rich” with respect to the content of an element in a phase refers to a content of the element in the phase of at least 50 at. % (e.g., at least 60 at. %, at least 70 at. %, at least 80 at. %, at least 90 at. %, at least 99.%, or higher). The term “phase” is generally used herein to refer to a state of matter. For example, the phase can refer to a phase shown on a phase diagram.

According to certain embodiments, during the sintering, Ti has a first diffusivity in itself and a second diffusivity in a second phase rich in the second metal, the first diffusivity being larger (e.g., at least 1%, at least 5%, at least 10%, at least 25%, at least 50%, or at least 100% larger) than the second diffusivity.

The sintering may be conducted in a variety of suitable environments. In certain embodiments, the nanocrystalline particulates are in an inert atmosphere during the sintering process. The use of an inert atmosphere can be useful, for example, when reactive metals are employed in the nanocrystalline particulates. For example, Ti and Mg are reactive with each other in the presence of oxygen.

In some embodiments, the sintering is performed in an atmosphere in which at least 90 vol. %, at least 95 vol. %, at least 99 vol. %, or substantially all of the atmosphere is made up of an inert gas. The inert gas can be or comprise, for example, helium, argon, xenon, neon, krypton, combinations of two or more of these, or other inert gas(es).

In certain embodiments, oxygen scavengers (e.g., getters) may be included in the sintering environment. The use of oxygen scavengers can reduce the degree to which the metals are oxidized during the sintering process, which may be advantageous according to certain embodiments. In some embodiments, the sintering environment can be controlled such that oxygen is present in an amount of less than 1 vol. %, less than 0.1 vol. %, less than 100 parts per million (ppm), less than 10 ppm, or less than 1 ppm.

According to certain embodiments, the sintering is conducted essentially free of external applied stress. For example, in some embodiments, for at least 20%, at least 50%, at least 75%, at least 90%, or at least 98% of the time during which sintering is performed, the maximum external pressure applied to the nanocrystalline particulates is less than or equal to 2 MPa, less than or equal to 1 MPa, less than or equal to 0.5 MPa, or less than or equal to 0.1 MPa. The maximum external pressure applied to the nanocrystalline particulates refers to the maximum pressure applied as a result of the application of a force external to the nanocrystalline particulates, and excludes the pressure caused by gravity and arising between the nanocrystalline particulates and the surface on which the nanocrystalline particulates are positioned during the sintering process. Certain of the sintering processes described herein can allow for the production of relatively highly dense sintered ultra-fine and nanocrystalline materials even in the absence or substantial absence of external pressure applied during the sintering process. According to certain embodiments, the sintering may be a pressureless sintering process.

According to certain embodiments, at least one activator element may be present during the sintering process. The activator element may enhance the sintering kinetics of Ti. According to certain embodiments, the activator element may provide a high diffusion path for the Ti atoms. For example, in some embodiments, the activator element atoms may surround the Ti atoms and provide a relatively high transport diffusion path for the Ti atoms, thereby reducing the activation energy of diffusion of the Ti. In some embodiments, this technique is referred to as activated sintering. The activator element may, in some embodiments, lower the temperature required to sinter the nanocrystalline particulates, relative to the temperature that would be required to sinter the nanocrystalline particulates in the absence of the activator element but under otherwise identical conditions. Thus, the sintering may involve, according to certain embodiments, a first sintering temperature, and the first sintering temperature may be lower than a second sintering temperature needed for sintering the Ti in the absence of the second metal. To determine the sintering temperature needed for sintering the Ti in the absence of the second metal, one would prepare a sample of the Ti material that does not contain the second metal but is otherwise identical to the nanocrystalline particulate material. One would then determine the minimum temperature needed to sinter the sample that does not include the second metal. In some embodiments, the presence of the second metal lowers the sintering temperature by at least 25° C., at least 50° C., at least 100° C., at least 200° C., or more.

According to certain embodiments, at least one stabilizer element may be present during the sintering process. The stabilizer element may be any element capable of reducing the amount of grain growth that occurs, relative to the amount that would occur in the absence of the stabilizer element but under otherwise identical conditions. In some embodiments, the stabilizer element reduces grain growth by reducing the grain boundary energy of the sintered material, and/or by reducing the driving force for grain growth. The stabilizer element may, according to certain embodiments, exhibit a positive heat of mixing with the sintered material. The stabilizer element may stabilize nanocrystalline Ti by segregation in the grain boundaries. This segregation may reduce the grain boundary energy, and/or may reduce the driving force against grain growth in the alloy.

In some embodiments, the stabilizer element may also be the activator element. The use of a single element both as the stabilizer and activator elements has the added benefit, according to certain embodiments, of removing the need to consider the interaction between the activator and the stabilizer. In some embodiments, the element that may be utilized as both the activator and stabilizer element may be a metal element, which may be any of the aforedescribed metal elements.

According to certain embodiments, when one element cannot act as both the stabilizer and the activator, two elements may be employed. The interaction between the two elements may be accounted for, according to some embodiments, to ensure that the activator and stabilizer roles are properly fulfilled. For example, when the activator and the stabilizer form an intermetallic compound each of the elements may be prevented from fulfilling their designated role, in some cases. As a result, activator and stabilizer combinations with the ability to form intermetallic compounds at the expected sintering temperatures should be avoided, at least in some instances. The potential for the formation of intermetallic compounds between two elements may be analyzed with phase diagrams.

According to one set of embodiments, titanium powders and magnesium powders (e.g., 10, 20, or 30 at. % Mg with the balance being titanium) can be mechanically alloyed via ball milling, cold compressed, and subsequently annealed (e.g., in a thermomechanical analyzer for several hours). In some embodiments, the Ti—Mg alloy system exhibits nanocrystalline grain size stabilization by formation of a nano-duplex structure.

According to certain embodiments, powders of elemental Ti and Mg are mixed and milled to achieve supersaturation and a decrease of the grain size to the nanometer scale. In some embodiments, annealing of compressed powders leads to the development of a nano-duplex structure consisting of Ti-rich grains and Mg-rich precipitates. In some embodiments, a nanocrystalline structure with grain sizes of around 110 nm can be maintained even after 8 hours at 500° C. (which is 84% of the melting temperature for Mg and 30% for Ti). In some embodiments, high relative densities can be achieved for Ti-20 at. % Mg and Ti-30 at. % Mg. It is believed that this may indicate that accelerated densification is possible.

U.S. Provisional Application No. 62/384,232, filed Sep. 7, 2016, and entitled “Stable Nano-Duplex Titanium-Magnesium Alloys” is incorporated herein by reference in its entirety for all purposes.

The following example is intended to illustrate certain embodiments of the present invention, but does not exemplify the full scope of the invention.

Example

This example demonstrates how processing by low-temperature, accelerated sintering methods were able to be applied to produce nanocrystalline titanium-magnesium (Ti—Mg) alloys with thermal stability and high relative density.

Titanium powders with different additions of magnesium powders (10, 20, and 30 at. % Mg) were mechanically alloyed via high-energy ball milling in a stainless steel vial and stainless steel media. With this process, supersaturated powders with microcrystalline particles and nanocrystalline grain sizes were produced after milling times of around 15 hours. The powders were then cold compressed and subsequently sintered in pure argon atmosphere. The microstructure of the milled powders consisted of supersaturated titanium grains with sizes of around 10 to 20 nm. After sintering (also referred to herein as “annealing”) to 600° C., the grain size increased to around 100 nm and separated into titanium-rich and magnesium-rich grains. Even after prolonged sintering times, the structure remained stable.

Accelerated sintering (pressureless) of nanocrystalline alloys was conducted. Production of supersaturated powders was accomplished via high-energy ball milling. It is believed that the sintering involved precipitation and neck formation of solute on solvent. The effect of necks may have involved fast solute diffusion due to excess vacancies, and diffusion of solvent within necks due to solubility of solvent in solute, that resulted in enhanced densification.

FIG. 2 shows the enthalpy of segregation, ΔH^(seg) (kJ/mol), and the enthalpy of mixing, AB′ (kJ/mol) of various metals with titanium. Magnesium (an alkaline earth metal) was determined to be a good candidate to alloy with titanium along with scandium (Sc) and yttrium (Y) (transition metals), thorium (Th) (an actinide), lanthanum (La) (a lanthanide), chromium (Cr), silver (Ag), iron (Fe), manganese (Mn), copper (Cu), and lithium (Li). This is because a positive enthalpy of mixing led to phase separation and a positive enthalpy of segregation led to grain boundary segregation, which stabilized the nanocrystalline structure. As shown in FIG. 2, Mg is in the nano-duplex region with Ti for nano-phase separation in a solvent-rich and solute-rich phase. A Ti—Mg phase diagram showed a large miscibility gap (not shown). The melting point of Mg is 650° C., much less than the melting temperature of Ti at 1668° C.

FIG. 3 shows a series of x-ray diffraction (XRD) spectra for nanocrystalline powder samples that contained titanium and 20 at. % magnesium (Ti-20 at. % Mg) that were processed by high-energy ball milling at 1000 cycles per minute for 0 hours, 2 hours, 4 hours, 8 hours, 12 hours, 16 hours, and 20 hours, using 5 grams of Ti—Mg mixture plus 1 wt. % stearic acid. Ti peaks moved to lower angles and Mg peaks disappeared, which demonstrated the supersaturation of Mg in Ti of the powders during milling. There was dissolution of magnesium in titanium. XRD patterns after 0 to 20 hours milling demonstrated supersaturation of Ti-20 at. % Mg powder during milling and a decrease of the grain size (peak shift to lower angles and peak broadening). FIG. 4 shows a distinct decrease of grain size below 20 nm after 16 hours milling and increase of the lattice parameters c and a for all mixtures. In addition, FIG. 4 demonstrates that a supersaturated phase was formed.

Processing of Ti-xMg was done with x=10 at. %, 20 at. %, and 30 at. %. High-energy ball milling of mixed elemental powders Ti-xMg (x=10 at. %, 20 at. %, 30 at. %) was conducted to produce supersaturated powders.

FIG. 4 shows a plot of grain sizes of nanocrystalline powders that contained titanium and 10 at. % Mg, 20 at. % Mg, and 30 at. % Mg measured from x-ray diffraction (XRD) and transmission electron microscopy (TEM) that were made by high-energy ball milling at 1000 cycles per minute for 0 hours, 2 hours, 4 hours, 8 hours, 12 hours, 16 hours, and 20 hours, using 5 grams of Ti—Mg mixture plus 1 wt. % stearic acid. Milling recipe: using steel vial and media, ball-to-powder ratio 10:1, 1 weight percent (wt. %) stearic acid, and milling time: e.g. 20 hours. As can be seen in FIG. 4, grain size decreased drastically as milling time increased.

FIG. 5 shows a series of TEM images and corresponding electron diffraction patterns for nanocrystalline powders that contained titanium and 10 at. % Mg, 20 at. % Mg, and 30 at. % Mg that were made by high-energy ball milling at 1000 cycles per minute for 20 hours, using 5 grams of Ti—Mg mixture plus 1 wt. % stearic acid. As indicated in FIG. 4, average grain sizes for these powders were 18 nm for 10 at. % Mg, 15 nm for 20 at. % Mg, and 10 nm for 30 at. % Mg, as measured by image analysis. The scale bar in all TEM images in FIG. 5 is 30 nm. The continuous rings in the electron diffraction patterns are characteristic of nanocrystalline samples in general.

FIG. 6 shows an electron diffraction pattern from TEM of Ti-20 at. % Mg after high-energy ball milling as described above, and diffraction rings of supersaturated titanium with Miller-Bravais indices (10-10), (0002), (10-11), (10-12), and (10-20) (corresponding to a hexagonal close-packed crystal structure) have been superimposed on the pattern for emphasis. Table 1 shows: d is the distance between atomic planes; d_(calculated) was calculated with the lattice parameter using the Bragg equation; and d_(measured) was measured in the diffraction pattern.

TABLE 1 Diffraction rings of supersaturated titanium. Miller-Bravais Index d_(calculated) d_(measured) (10-10) 0.221 0.221 (0002) 0.185 0.205 (10-11) 0.171 0.195 (10-12) 0.101 0.148 (10-20) 0.073 0.128

Compacted samples were sintered in a thermomechanical analyzer (Netzsch), using standard parameters for the instrument. Sintering of cold compressed samples (h=4 mm, d=6 mm) was carried out in the thermomechanical analyzer, isothermally held at e.g. 500° C. for 8 hours to determine the stability of the nanocrystalline structure, and with a constant heating rate of e.g. 5 K/min to e.g. 550° C. to explore the sintering behavior. First, cold compression of powders was carried out with different loads (1t-6t), where the conversion from t to MPa is tabulated in Table 2.

TABLE 2 Conversion from t to MPa. Resulting Pressure Applied Load [t] [MPa] 1 347 2 694 3 1041 4 1388 5 1735 6 2058

A representative sample size had a height of approximately 4 mm, and a diameter of approximately 6 mm. Samples were covered with tantalum (Ta) foil (FIG. 7A) or copper (Cu) tube (FIG. 7B). Sintering under isothermal conditions and with constant heating rate was carried out. The isothermal condition was at from 400° C. to 600° C. (e.g., 500° C.) for 8 h. The constant heating rate condition was a heating rate of from 5 K/min to 20 K/min (e.g., 5 K/min) to a maximum temperature of from 550° C. to 700° C.

FIG. 8 demonstrates the effect of the applied load during cold compression (in t) on the relative density (%) of the nanocrystalline alloy Ti with 20 at. % Mg. Green bodies were only pressed at room temperature, and sintered samples were sintered using a ramp rate of 5 K/min to 600° C. The powders had first been high-energy ball milled at 1000 cycles per minute for 20 hours, using 5 grams of Ti—Mg mixture plus 1 wt. % stearic acid. The relative density was measured using dimensions of the sample and then calculated using the theoretical density of the sample. As FIG. 8 demonstrates, compaction to greater than 80% relative density was achieved in green bodies and greater than 95% relative density in sintered samples.

Table 3 shows the melting temperature of titanium and magnesium alone (T_(m)), half of the melting temperature of Ti and Mg where the half-melt temperature was calculated by first converting to Kelvin (0.5·T_(m)), and room temperature relative to the melting temperature of titanium and magnesium where the calculation was made by first converting to Kelvin (RT). Table 3 shows that compared to ordinary sintering, the sintering temperatures used in this example are very low.

TABLE 3 Melting temperatures and related temperatures for titanium and magnesium. 0.5 · T_(m) [° C.]; RT (25° C.); determined using determined using T_(m) [° C.] Kelvin Kelvin Ti 1668 698 0.15 · T_(m) Mg 650 189 0.32 · T_(m)

FIG. 9 shows the in situ progression of the relative density (%) for samples sintered to from 550° C. to 600° C. for different compositions of Ti—Mg alloys. The final relative density of the alloy depended at least in part on composition, compaction pressure during cold compression of the Ti—Mg powder, and sintering temperature. The deviation of the curves can be attributed to the Ta foil in which the samples were encased being rigid.

The microstructure after sintering for the Ti—Mg alloys was analyzed by scanning transmission electron microscopy-energy dispersive x-ray spectroscopy (STEM-EDS). FIGS. 10A-10C show STEM-EDS of Ti-20 at % Mg alloy after having sintered at 500° C. for 8 h (scale bar 600 nm). The EDS map of Ti (FIG. 10B) shows that the titanium was concentrated primarily in the light gray continuous region of the STEM image (FIG. 10A). The EDS map of Mg (FIG. 10C) shows that the magnesium was concentrated primarily in the black isolated regions of the STEM image (FIG. 10A).

FIG. 11 shows an XRD pattern comparison before (dotted) and after (solid lines) sintering. The Ti peaks shifted back in the direction of pure Ti and narrowed. Small Mg peaks occurred after sintering, which agreed with the occurrence of a Mg-rich phase and some grain growth depicted in the STEM results.

FIGS. 12, 13, and 10A show STEM images of different Ti—Mg alloy compositions after sintering at 500° C. for 8 hours in pure Ar atmosphere and STEM-EDS images of the Ti and Mg distribution for Ti-20 at. % Mg. FIG. 12 (Ti-10 at. % Mg, d=119 nm, scale bar=300 nm), FIG. 13 (Ti-30 at. % Mg, d=126 nm, scale bar=300 nm), and FIG. 10A (Ti-20 at. % Mg, d=107 nm, scale bar=300 nm), are the STEM images, and FIGS. 10B and 10C are the STEM-EDS images for Ti-20 at. % Mg. The grain size was stabilized at on average 110 nm and the grain structure of all three samples showed a well-developed nano-duplex structure comprising Ti-rich grains and Mg-rich precipitates, shown by the element distribution in the STEM-EDS images.

Table 4 shows the grain size after sintering for Ti-10 at. % Mg, Ti-20 at. % Mg, and Ti-30 at. % Mg alloys. In addition, Table 4 indicates the change in relative density between the cold compressed powder and the resulting sintered alloy. Grain sizes were determined by TEM and XRD. FIG. 9 shows the change of the relative density during sintering with a constant heating rate of 5 K/min to 550° C. for different Ti—Mg alloys. A distinct densification above 350° C. occurred for Ti-20 at. % Mg and Ti-30 at. % Mg. Cold compression led to higher relative densities than expected. Relative densities of greater than 90% after sintering were achieved.

TABLE 4 Relative Density and Grain Size Data for Ti-Mg Alloys. Relative Grain Holding density Relative Size Cold Heating Final Time at after density after Compression Rate Temp. Temp. sintering Change Sintering (MPa) (K/min) (° C.) (hours) (%) [%] [nm] Ti-10 Mg 1735 40 500 8 78.5 −1.9 119 Ti-10 Mg 347 40 500 8 63.2 −0.6 144 Ti-10 Mg 347 5 550 0 60.6 −0.6 87 Ti-20 Mg 347 40 400 8 87.6 4.5 53 Ti-20 Mg 1735 40 500 8 89.0 5.6 107 Ti-20 Mg 347 40 500 8 67.6 7.6 149 Ti-20 Mg 694 40 500 24 78.4 13.1 185 Ti-20 Mg 347 40 600 6 73.8 12.1 270 Ti-20 Mg 347 40 600 24 76.3 14.5 385 Ti-20 Mg 1735 5 550 0 91.9 8.0 109 Ti-20 Mg 347 5 550 0 60.8 2.6 92 Ti-20 Mg 1735 5 550 0 90.0 7.2 Ti-20 Mg 1735 5 600 0 91.7 8.6 Ti-20 Mg 2082 5 600 0 89.6 7.7 Ti-20 Mg 1735 5 700 0 93.9 12.9 Ti-20 Mg 347 5 700 0 78.6 14.0 242 Ti-20 Mg 347 10 800 0 79.9 16.4 Ti-30 Mg 1735 40 500 8 93.5 5.8 126 Ti-30 Mg 347 40 500 8 59.9 8.9 144 Ti-30 Mg 694 5 550 0 63.4 7.8 105 Ti-30 Mg 2082 5 600 0 96.1 7.0 157 Ti-30 Mg 347 10 550 0 69.4 8.0 Ti-30 Mg 347 5 600 0 74.4 14.1 119 Ti-30 Mg 347 10 600 0 77.4 13.2 113 Ti-30 Mg 347 15 600 0 75.5 10.6 90 Ti-30 Mg 347 20 600 0 75.0 12.4 95

While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the functions and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings of the present invention is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, and/or methods, if such features, systems, articles, materials, and/or methods are not mutually inconsistent, is included within the scope of the present invention.

The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified unless clearly indicated to the contrary. Thus, as a non-limiting example, a reference to “A and/or B,” when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A without B (optionally including elements other than B); in another embodiment, to B without A (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03. 

What is claimed is:
 1. A sintered nanocrystalline metal alloy, comprising: Ti; and a second metal; wherein: Ti is the most abundant metal by atomic percentage in the sintered nanocrystalline metal alloy, and the sintered nanocrystalline metal alloy has a relative density of at least 80%.
 2. A sintered nanocrystalline metal alloy, comprising: Ti; and a second metal; wherein: the second metal and Ti exhibit a miscibility gap, and the sintered nanocrystalline metal alloy has a relative density of at least 80%.
 3. A bulk nanocrystalline metal alloy, comprising: Ti; and a second metal; wherein: Ti is the most abundant metal by atomic percentage in the bulk nanocrystalline metal alloy, and the bulk nanocrystalline metal alloy is substantially stable at a temperature that is greater than or equal to 100° C.
 4. A bulk nanocrystalline metal alloy, comprising: Ti; and a second metal; wherein: Ti is the most abundant metal by atomic percentage in the bulk nanocrystalline metal alloy, and the bulk nanocrystalline metal alloy has an average grain size of less than 300 nm.
 5. A metal alloy, comprising: Ti; and Mg; wherein the metal alloy has a relative density of greater than or equal to 80%.
 6. The metal alloy of any one of the preceding claims, wherein the second metal comprises an alkaline earth metal.
 7. The metal alloy of any one of the preceding claims, wherein the second metal is selected from the group consisting of Mg, La, Y, Th, Sc, Cr, Ag, Fe, Mn, Cu, and Li.
 8. The metal alloy of any one of the preceding claims, wherein the second metal is Mg.
 9. The metal alloy of any one of the preceding claims, wherein the second metal is present in the nanocrystalline metal alloy in an amount of less than 40 atomic percent of the nanocrystalline metal alloy.
 10. The metal alloy of any one of the preceding claims, wherein the second metal is present in the nanocrystalline metal alloy in an amount of from 1 atomic percent to 40 atomic percent of the nanocrystalline metal alloy.
 11. The metal alloy of any one of the preceding claims, wherein the second metal is present in the nanocrystalline metal alloy in an amount of from 8 atomic percent to 32 atomic percent of the nanocrystalline metal alloy.
 12. The metal alloy of any one of the preceding claims, wherein Ti is present in the nanocrystalline alloy in an amount of at least 50 atomic percent.
 13. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy is a dual-phase metal alloy.
 14. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy further comprises a third metal.
 15. The metal alloy of any one of the preceding claims, wherein the second metal and Ti exhibit a miscibility gap.
 16. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy has an average grain size of less than 300 nm.
 17. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy has an average grain size of less than 150 nm.
 18. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy has an average grain size of less than 125 nm.
 19. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy has an average grain size of less than 100 nm.
 20. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy has a relative density of at least 80%.
 21. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy has a relative density of at least 85%.
 22. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy has a relative density of at least 90%.
 23. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy has a relative density of at least 95%.
 24. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy has a relative density of at least 97%.
 25. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy has a relative density of at least 99%.
 26. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy is a bulk nanocrystalline metal alloy.
 27. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy is substantially stable at a temperature that is greater than or equal to 100° C.
 28. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy is substantially stable at a temperature that is greater than or equal to 300° C.
 29. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy is substantially stable at a temperature that is greater than or equal to 500° C.
 30. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy is substantially stable at a temperature that is greater than or equal to 700° C.
 31. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy is substantially stable at a temperature that is greater than or equal to 800° C.
 32. The metal alloy of any one of the preceding claims, wherein Ti is at least partially soluble in the second metal.
 33. The metal alloy of any one of the preceding claims, wherein Ti and the second metal are in a solid solution.
 34. The metal alloy of any one of the preceding claims, wherein the nanocrystalline metal alloy has a first grain size and a sintered material comprising Ti in the absence of the second metal has a second grain size, the first grain size being smaller than the second grain size.
 35. A method of forming a nanocrystalline metal alloy, comprising: sintering a plurality of nanocrystalline particulates to form the nanocrystalline metal alloy; wherein at least some of the nanocrystalline particulates comprise Ti and a second metal, and Ti is the most abundant metal by atomic percentage in at least some of the nanocrystalline particulates.
 36. A method of forming a nanocrystalline metal alloy, comprising: sintering a plurality of nanocrystalline particulates to form the nanocrystalline metal alloy; wherein: at least some of the nanocrystalline particulates comprise Ti and a second metal; and sintering the plurality of nanocrystalline particulates involves heating the nanocrystalline particulates to a first sintering temperature that is greater than or equal to 300° C. and less than or equal to 850° C. for a sintering duration greater than or equal to 10 minutes and less than or equal to 24 hours.
 37. A method of forming a nanocrystalline metal alloy, comprising: sintering a plurality of nanocrystalline particulates to form the nanocrystalline metal alloy; wherein: at least some of the nanocrystalline particulates comprise Ti and a second metal; and sintering the plurality of nanocrystalline particulates involves heating the nanocrystalline particulates such that the nanocrystalline particulates are not at a temperature of greater than or equal to 1200° C. for more than 24 hours.
 38. A method of forming a nanocrystalline metal alloy, comprising: sintering a plurality of nanocrystalline particulates to form the nanocrystalline metal alloy; wherein: at least some of the nanocrystalline particulates comprise Ti and a second metal; Ti is the most abundant metal by atomic percentage in at least some of the nanocrystalline particulates; and the sintering comprises heating the nanocrystalline particulates to a first sintering temperature lower than a second sintering temperature needed for sintering Ti in the absence of the second metal.
 39. A method of forming a nanocrystalline metal alloy, comprising: sintering a plurality of nanocrystalline particulates to form the nanocrystalline metal alloy; wherein: at least some of the nanocrystalline particulates comprise Ti and a second metal; and the second metal and Ti exhibit a miscibility gap.
 40. A method of forming a nanocrystalline metal alloy, comprising: sintering a plurality of nanocrystalline particulates to form the nanocrystalline metal alloy; wherein: at least some of the nanocrystalline particulates comprise Ti and a second metal; Ti is the most abundant metal by atomic percentage in at least some of the nanocrystalline particulates; and the nanocrystalline metal alloy has a relative density of at least 80%.
 41. A method of forming a metal alloy, comprising: sintering powder comprising Ti and Mg to produce the metal alloy, wherein the metal alloy has a relative density of greater than or equal to 80%.
 42. The method of any one of claims 35-41, wherein the Ti and the second metal are present in a non-equilibrium phase.
 43. The method of any one of claims 35-42, wherein the non-equilibrium phase undergoes decomposition during the sintering.
 44. The method of any one of claims 35-43, wherein the non-equilibrium phase undergoes decomposition during the sintering, and the decomposition of the non-equilibrium phase accelerates a rate of sintering of the nanocrystalline particulates.
 45. The method of any one of claims 35-44, wherein the non-equilibrium phase comprises a supersaturated phase comprising the second metal dissolved in Ti.
 46. The method of any one of claims 35-45, wherein the second metal comprises an alkaline earth metal.
 47. The method of any one of claims 35-46, wherein the second metal is selected from the group consisting of Mg, La, Y, Th, Sc, Cr, Ag, Fe, Mn, Cu, and Li.
 48. The method of any one of claims 35-47, wherein the second metal is Mg.
 49. The method of any one of claims 35-48, further comprising cold pressing the plurality of nanocrystalline particulates during at least one portion of time prior to the sintering.
 50. The method of any one of claims 35-49, wherein the cold pressing comprises cold compression of the plurality of nanocrystalline particulates at a force greater than or equal to 300 MPa.
 51. The method of any one of claims 35-50, wherein the cold pressing comprises cold compression of the plurality of nanocrystalline particulates at a force greater than or equal to 300 MPa and less than or equal to 2500 MPa.
 52. The method of any one of claims 35-51, wherein the sintering comprises heating the nanocrystalline particulates to a first sintering temperature lower than a second sintering temperature needed for sintering Ti in the absence of the second metal.
 53. The method of any one of claims 35-52, wherein the sintering comprises heating the nanocrystalline particulates to a temperature greater than or equal to 300° C. and less than or equal to 850° C. for a duration greater than or equal to 10 minutes and less than or equal to 24 hours.
 54. The method of any one of claims 35-53, wherein the sintering comprises heating the nanocrystalline particulates to a temperature greater than or equal to 300° C. and less than or equal to 850° C. for a duration greater than or equal to 20 minutes and less than or equal to 3 hours.
 55. The method of any one of claims 35-54, wherein the sintering comprises heating the nanocrystalline particulates to a temperature greater than or equal to 300° C. and less than or equal to 450° C. for a duration greater than or equal to 20 minutes and less than or equal to 3 hours.
 56. The method of any one of claims 35-55, wherein the first sintering temperature is lower than or equal to 700° C.
 57. The method of any one of claims 35-56, wherein the non-equilibrium phase comprises a solid solution.
 58. The method of any one of claims 35-57, further comprising forming at least some of the nanocrystalline particulates by mechanically working a powder comprising Ti and the second metal.
 59. The method of any one of claims 35-58, further comprising forming at least some of the nanocrystalline particulates by ball milling a powder comprising Ti and the second metal.
 60. The method of any one of claims 35-59, wherein at least some of the nanocrystalline particulates have a grain size of smaller than or equal to 50 nm.
 61. The method of any one of claims 35-60, wherein at least some of the nanocrystalline particulates have a grain size of greater than or equal to 5 nm and smaller than or equal to 25 nm.
 62. The method of any one of claims 35-61, wherein at least some of the nanocrystalline particulates have a grain size of greater than or equal to 10 nm and smaller than or equal to 20 nm.
 63. The method of any one of claims 35-62, wherein the second metal is present in the nanocrystalline particulates in an amount of less than or equal to 40 atomic percent of the nanocrystalline particulates.
 64. The method of any one of claims 35-63, wherein the second metal is present in the nanocrystalline particulates in an amount of greater than or equal to 8 atomic percent and less than or equal to 32 atomic percent of the nanocrystalline particulates.
 65. The method of any one of claims 35-64, wherein the nanocrystalline metal alloy further comprises a third metal material
 66. The method of any one of claims 35-65, wherein the nanocrystalline metal alloy has a relative density of at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, or at least 99%.
 67. The method of any one of claims 35-66, wherein the sintering further comprises forming a second phase at at least one of a surface and a grain boundary of the nanocrystalline particulates during the sintering; and Ti is insoluble in the second phase.
 68. The method of any one of claims 35-67, wherein the sintering further comprises forming a second phase at at least one of a surface and a grain boundary of the nanocrystalline particulates during the sintering; and the second phase is rich in the second metal.
 69. The method of any one of claims 35-68, wherein the nanocrystalline metal alloy has a first grain size and a sintered material comprising Ti in the absence of the second metal has a second grain size, the first grain size being smaller than the second grain size.
 70. The method of any one of claims 35-69, wherein the nanocrystalline metal alloy is substantially stable at a temperature that is greater than or equal to 100° C., at a temperature that is greater than or equal to 200° C., at a temperature that is greater than or equal to 300° C., at a temperature that is greater than or equal to 500° C., at a temperature that is greater than or equal to 600° C., and/or at a temperature that is greater than or equal to 700° C.
 71. The method of any one of claims 35-70, wherein the sintering is conducted essentially free of external applied stress.
 72. A nanocrystalline metal alloy produced by the method of any one of claims 35-71. 